The activation of immune function in T cells requires the simultaneous binding by a specific receptor of antigen and a transmembrane glycoprotein coded for by the major histocompatibility complex. As with other cell surface receptors, there is indirect evidence that cross-linking of T cell receptors precedes activation. That is, cross-linking by antibody, in the absence of antigen, is sufficient to stimulate the functional response. Experiments are described in this proposal are aimed at determining whether significant receptor-induced accumulation of antigen occurs in the contact region between a T cell and a target membrane. The target membrane will consist of a planar phospholipid bilayer supported on glass into which MHC antigens have been reconstituted. Supported membranes previously have been shown to be effective in the stimulation of immune function from MHC restricted, antigen specific T cells. Before undertaking experiments to detect redistribution of membrane components, effort will be concentrated on correcting certain shortcomings of the supported membranes, particularly with a view toward producing supported membranes in which the MHC molecules are laterally mobile. To increase the efficiency of reconstitution, we shall try several approaches to inserting MHC antigens into preformed supported bilayers. Functional assays will be used to determine the effects of variation in the lateral mobility of MHC molecules. For cytotoxic T cells recognizing class I molecules, a number of functional responses will be tried, especially induction of gamma-interferon, that will allow studies with model membranes to be extended to this T cell subset. Fluorescence microscopy will be used to measure the degree of redistribution of fluorescently tagged, laterally mobile MHC molecules. The degree of redistribution should reflect the affinity of the receptor for the MHC ligand. The results of these experiments should have general implications for receptors participating in cell-cell recognition in other biological and developmental systems.